In Arma 2, items from the backpack could not be immediately accessed unless the player took them out of the backpack onto his vest or uniform.

In Arma 3, however, any item from the backpack is as readily available as from the vest or uniform. This includes throwing grenades, reloading magazines, etc. Since the backpack is usually, well, on the players back, he should not have immediate access to it.

The (negative) result of the immediate accessibility are:

• Rocket spamming - since Missile/Rocket launchers reload way too fast anyway, spamming a backpack full of rockets is easy.
• Grenade Spamming - Since volume and weight in Arma 3 are both rolled into an (artificial) loadout value, spamming rifle grenades from the backpack is very easy, and the backpack can take a metric ton of these things.
• Backpacks have no disadvantage - since there is no collusion with backpacks and weapons, and the backpack itself weights next to nothing, there is no tactical decision making involved. Taking a backpack is always better than none.
• Quick magazine reloading - The player can have zillions of magazines and does not need to husband anything since they are readily available.